Electroplating bath and process



United States Patent 3,454,475 ELECTROPLATING BATH AND PROCESS John D. Rushmere, Grand Island, and William H. Todd, Lewiston, N.Y., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed July 13, 1965, Ser. No. 471,753 Int. Cl. C23b /10, 5/46 US. Cl. 204-55 4 Claims ABSTRACT OF THE DISCLOSURE An aqueous, alkaline, cyanide zinc electroplating bath and process uses a water-soluble homopolymer of {it-alanine as a brightening agent.

This invention relates to the electrodeposition of zinc and, more particularly, to electrodepositing zinc from alkaline cyanide plating baths containing a polymeric brightening agent.

In electrodepositing zinc from alkaline cyanide plating baths, it has been found necessary to include in the bath a quantity of some material which facilitates the deposition of bright zinc deposits. Various materials have been used with varying degrees of success. Some are extremely expensive and others must be used in large quantities to be effective or suffer from instability in the plating bath.

The primary object of this invention is to provide a new polymeric material useful for depositing bright zinc from alkaline cyanide baths. A further object is to provide an additive which may be used effectively in small quantities and which has a relatively low cost. Other objects will be apparent from the detailed description which follows.

The objects of this invention are accomplishd by the use of a low molecular weight, water-soluble homopolymer of fi-alanine. These polymers are linear, straight chain polymers having repeating units which may be represented by the fomula:

The polymers can be prepared by the process described in US. Patent 2,734,081 or by another process which will be described later herein. The preferred polymers are completely miscible in water and are represented by those in which n is an integer from 5 to 10. However, the principal requirement resides in the polymers having at least some solubility in the cyanide plating bath. Since the polymers are highly active brightening agents, the required solubility may be very low, i.e., 0.01 weight percent or less.

In carrying out the process of the present invention, a small quantity of the Si-alanine homopolymer is introduced into a plating bath containing zinc cyanide. The homopolymer is preferably introduced into the bath from a stock solution. The solution may be aqueous or contain a water-miscible organic solvent such as methyl Cellosolve, isopropanol, or other organic solvent which is inert, i.e., non-reactive, with respect to the homopolymer and bath constituents. The amount of the polymer used is not critical but should generally be sufiicient to provide from about 0.1 to 20 grams/liter in the bath. The optimum amount will vary depending on the type of plate desired, the method of plating used and the current density employed in the particular plating run. In one embodiment of this invention, a plating bath containing zinc cyanide, sodium hydroxide, sodium cyanide, sodium carbonate and sodium sulfide is prepared by dissolving the 3,454,475 Patented July 8, 1969 "ice constituents in water. The poly-fi-alanine is then added separately from a concentrated aqueous brightener solution. The concentrate may contain from about 5 to 25% by Weight of the polymer.

The invention will be further illustrated by the following examples in which parts and percentages are by weight unless otherwise specified. The poly-B-alanine used in Example I was prepared by refiuxing 100 grams of ethylene cyanohydrin, 40 grams of cyclohexane, 18 grams of 1,2- dimethoxyethane and 2 grams of potassium hydroxide. After polymerization, which required about two hours, 55 grams of solvent were decanted. The poly-,B-alanine residue was a hard, sticky yellow mass.

EXAMPLE I An aqueous plating bath having the following composition was used to plate a steel panel in a Hull cell.

Grams/liter Zinc cyanide 60 Sodium hydroxide 90 Sodium cyanide 40 Sodium carbonate Sodium sulfide 1 The plating operation was carried out at room temperature, i.e., between about 25 and 30 C. Two grams of poly-fl-alanine, prepared as described previously, was dis solved in 20 cc. of water to form a stock solution. An amount of the stock solution suflicient to provide 0.4 gram/liter of poly-fi-alanine was added to the plating bath. The steel panel was plated in the Hull cell using a current of 2 amps for 5 minutes. The panel was removed from the plating bath, dipped in 0.5 weight percent of nitric acid to remove surface stains, washed and then dried. The panel had the following characteristics:

Current density (amps/ft?) Plate characteristics 2-10 Dull plate. 10-60 Bright plate. 60-150 Bright plate with some haze.

The plating operation just described was repeated except that the quantity of poly-,B-alanine in the bath was increased to 0.8 gram/liter. The following results were obtained:

Current density (amps/ft?) Plate characteristics 2-15 Dull plate. 15-50 Very bright plate. 50-150 Bright plate with some haze.

From the foregoing plating operations it can be seen that by increasing the amount of poly-fi-alanine in the bath an increase in brightness and quality of plate in the mid-current density range was obtained.

The plating operation described above was again repeated as a control experiment in which the poly-B-alanine was omitted. The best regions of the panel were inferior to the regions described as bright plate with some haze in the tests previously described.

EXAMPLE II methyl Cellosolve solution was successively increased in stages as follows: 0.8 milliliter/liter, 1.6 milliliters/liter, 2.4 milliliters/liter, 3.2 milliliters/liter. After bright dipping in dilute nitric acid solution to remove stains, the quality of the zinc plate showed a successive improvement over the plate obtained in the absence of the additive. At the highest concentration, 3.2 milliliters/liter, the zinc plate was very bright on all surfaces of the crank hole cover and had the appearance of a product obtainable from the best known commercially available brighteners.

EXAMPLE III In this example, methyl Cellosolve was used in preparing the poly-B-alanine and a barrel plating operation was carried out. The poly-B-alanine was prepared by refluxing 100 grams of ethylene cyanohydrin, 20 grams of methyl Cellosolve and 2 grams of sodium cyanide. The resulting reflux, which contained approximately 80 weight percent of poly-,B-alanine, was added directly to the bath. 100 grams of No. 8 by %-inch steel screws were barrel plated in a bath having the composition described in Example I containing 4.5 milliliters/liter of the poly-5- alanine/methyl Cellosolve solution. The screws were plated for minutes in a Daniels Type 3H minibarrel using a current of 5 amps. After washing and bright dipping in 0.5 weight percent nitric acid, the screws were found to have a blue-white color and were bright and lustrous.

EXAMPLE IV In this example, isopropanol was used as a solvent in preparing poly-,B-alanine and a still plating operation was carried out. The poly-B-alanine was prepared by refluxing 100 grams of ethylene cyanohydrin, 40 grams of isopropanol and 2 grams of potassium hydroxide. To the cold reflux, 160 grams of cold water was added to form a yellow stock solution having a specific gravity of 1.05. A bath having the composition of that described in Example I and containing 1.5 grams/liter of stock solution was prepared. A six link length of steel chain (each link 1-inch long by %-inch wide made from -inch rod) was suspended in the bath and plated for 15 minutes using a zinc oxide, rather than zinc" cyanide, may be used in the baths. Details for preparing and using the baths are, of course, well known to those skilled in the art. The bright zinc deposits provided by the present invention are responsive to treatment with protective agents. Excellent results are obtained using a variety of available commercial products such as chromate conversion coating preparations.

As many widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not to be limited to the specific embodiments thereof as defined in the appended claims.

We claim:

1. An aqueous, alkaline, cyanide zinc electroplating bath containing as a brightening agent therefor a Watersoluble homopolymer of 3-alanine, said homopolymer being present in an amount sufficient to yield a zinc electrodeposit of improved brightness.

2. The bath of claim 1 wherein said homopolymer is present in an amount from about 0.1 to 20 grams/liter of bath and has the structural formula:

wherein n is an integer from 5 to 10.

3. In the process for electrodepositing zinc from an aqueous, alkaline, cyanide plating bath, the improvement comprising adding to said bath from about 0.1 to 20 grams/liter of a water-soluble homopolymer of B-alanine.

4. The process of claim 4 wherein said ,B-alanine homopolymer is dissolved in a water-miscible organic solvent.

References Cited UNITED STATES PATENTS 2,451,426 10/1948 Bair et a1. 204- 2,589,209 3/ 1952 Kardos 20455 2,680,712 6/1954 Diggin et al. 204 55 2,691,643 10/ 1954 Chirtel et a1. 260112.5 2,734,081 2/ 1956 Boatright.

2,791,554 5/1957 Winters 204-55 2,798,040 7/1957 Pye et a1. 204-55 X R 2,860,089 11/1958 Jackson 204-55 2,989,449 6/1961 Mackey et a1. 204 55 JOHN H. MACK, Primary Examiner.

G. L. KAPLAN, Assistant Examiner. 

